Accumulating evidence implicates the neutrophil as playing an important role in myocardial injury following coronary artery occlusion and reperfusion. Although there are several mechanisms by which the neutrophil can produce myocardial damage, including mechanical plugging and release of oxidants, relatively little attention has been focused on the role of neutrophil-derived proteolytic enzymes in this tissue damage. There is abundant in-vitro evidence and accumulating data in other in-vivo organ systems to support the importance of this proteolytic enzyme mechanism. Moreover, recent data which demonstrates that neutrophil-derived oxidants can both inactivate antiproteinases responsible for regulating toxic proteinase and activate tissue-destructive metalloproteinases, suggest that the role of oxidants in myocardial damage may be more complex than originally considered. Accordingly, we will examine the broad hypothesis that the proteolytic enzymes released from triggered neutrophils contribute to myocardial injury following coronary occlusion and reperfusion. We will systemically investigate the role of neutrophil-derived proteolytic enzymes in myocardial injury by using synthetic or recombinant proteinase inhibitors directed against the four proteolytic enzymes most commonly implicated in tissue damage; namely, the serine proteinase, elastase and cathepsin G, and the metalloproteinases, collagenase and gelatinase. In addition, we will examine the role of the cytokine, interleukin-1 (IL-1), since the cytokine system has been implicated in mediating this proteolytic injury. Initial in-vitro analysis of canine neutrophil proteolytic potential and the inhibitory effect of each of the inhibitors will be carefully examined. Individual protocols will be designed to study each of: a specific elastase inhibitor; 2) a recombinant secretory leukoproteinase inhibitor (rSLPI); 3) collagenase and gelatinase inhibitors, and 4) IL-1 inhibitor. Following these in-vitro analyses to characterize the specific proteolytic mechanisms involved, a series of in-vivo studies will be performed in acute and chronic canine models of coronary occlusion and reperfusion. Experiments will designed: 1) to study the effects of a synthetic elastase inhibitor on myocardial damage and functional recovery following ischemia and reperfusion; 2) to examine the effects of rSLPI, a small molecular weight elastase and cathepsin G inhibitor; 3) to investigate the effects of metalloproteinase inhibitors, and 4) to examine the cardioprotection provided by a recombinant IL-1 receptor antagonist. End points for analysis in these experiments will include infarct size, ventricular function, tissue edema, and histopathologic tissue analysis. An understanding of the mechanisms of neutrophil-mediated myocardial injury will not only provide specific information concerning the pathogenesis of ischemic and reperfusion damage but may also point towards therapeutic interventions which may prove to be important adjuncts to thrombolytic therapy in man.